The objective of the proposed experiments is to increase the repair efficiency of damaged or degenerated neuronal circuits in the brain, focusing the interest on restoration of corticocortical or cortico-thalamic neurons of adult mice in vivo. Immature neurons and neural precursors will be transplanted in the cortex undergoing a defined and controllable targeted degeneration of selective neurons (neurodegeneration). This apoptotic neuronal degeneration will be biophysically induced by photoactivation of targeting molecules, causing specific degeneration only to desired subpopulations of neurons in vivo. In this project, survival and differentiation of new neurons will be modulated by sensory experience, which produces activity-dependent plastic changes in the somatosensory cortex of adult mice. The group of Prof. Macklis has shown that neurons can be replaced by immature neuroblasts transplanted into the cerebral cortex after targeted degeneration of selected neurons. These newly incorporated cells can differentiate appropriately into precisely the right kind of neuron, re-establish appropriate long-distance projections, develop a mature phenotype, and make appropriate connections with recipient circuitry. The Macklis lab also showed, for the first time, that neurogenesis can be induced in the adult mouse neocortex, and new neurons can thereby be recruited from endogenous precursors manipulated in situ, without transplantation. It is postulated that targeted neuronal cell death produces local reexpression of developmental signals responsible for directed neuronal migration, differentiation, and connectivity of transplanted neuroblasts, or either transplanted or endogenous precursors. Complementary to these results, there is growing evidence in the literature suggesting that endogenous proliferation and differentiation of neurons (neurogenesis), which in the intact adult mouse brain is observed to the hippocampus and olfactory bulb, can be regulated by experience. In the proposed research project, it will be evaluated whether the effectiveness of differentiation, integration, and survival within circuitry by transplanted neuroblasts in the somatosensory cortex of mice can be improved by experimental manipulations of their circuit activity. The proposed research will evaluate the effects of induction of activity-dependent plastic changes in the adult somatosensory cortex, achieved by sensory stimulation and sensory deprivation. Intense long-lasting sensory stimulation will be induced by housing animals in completely new "tactile-rich" area (enriched environment) or by intense forced movement of tactile whiskers (mechanical stimulation of vibrissae). Sensory deprivation will base on long-lasting tactile whisker trimming or vibrissectomy (restriction of sensory input to the cortex). The effects on survival, differentiation, and integration of transplanted neuroblasts in the adult cortex undergoing experimentally-induced neurodegeneration and a range of stimulation-dependent activation will be studied with a range of established immunocytochemical, neuroanatomical, and genetic tracer methods well established in the Macklis lab, analyzed using fluorescence and confocal microscopy to evaluate cellular integration, synaptic circuit formation, and trans-synaptic functional activation.